The U.S. congress has about six weeks to debate and vote on the Iran nuclear deal. Judging by the grilling Secretary of State John Kerry and his team are getting in House and Senate hearings, the deal’s proponents are having a hard time addressing at least four key problems.

• First, the agreement does not dismantle the country’s nuclear industry or deny Iran the right to enrich uranium; that battle is over.

The measures are designed instead to disassemble, restrict and confine important parts of the country’s nuclear program to achieve one specific goal: increase the timeline required for Iran to enrich a sufficient quantity of weapons-grade uranium to produce a nuclear bomb, its “breakout capacity.”

Iran’s current capacity stands at roughly two to three months, a truly remarkable achievement, given the crippling economic sanctions that were supposed to deter Iran from becoming a nuclear threshold power. The sanctions regime has obviously failed to prevent proliferation. There is no credible military alternative that could achieve the same concessions, and the status quo (ongoing proliferation despite sanctions) simply raises the prospects of a military confrontation.

Given these unappealing alternatives, negotiators rationally settled on a less ambitious goal of managing the risk by extending Iran’s breakout capacity to one year, which explains the second problem.

• The validity of the administration’s breakout estimates relies heavily on the International Atomic Energy Agency’s (IAEA) capacity to access crucial information about Iran’s current and past nuclear programs.

Now, negotiators did manage to get strong verification and transparency measures included in an earlier version of the Joint Comprehensive Plan of Action (JCPOA) that emerged from the Lausanne negotiations in April 2015.

In addition to 24-7 access to declared nuclear facilities through video monitoring, as stipulated in the standard Safeguard Agreement of the Non-Proliferation Treaty, Iran also agreed at the time to permanent implementation of the IAEA’s “Additional Protocol” that commits signatories to provide inspectors with expanded access to undeclared nuclear facilities or any military site suspected of playing a role in the country’s vast nuclear program.

However, the final agreement signed in July includes a key compromise allowing Iran to “provisionally” apply the Additional Protocol through managed access to undeclared military sites engaged in suspicions activity — Iran can take up to 24 days to negotiate visits to these contested locations.

If these efforts fail to resolve the impasse, subsequent negotiations in the UN Security Council regarding the scope of non-compliance and appropriate “snap back” measures can take even longer.

Without timely, intrusive and unfettered access to military sites and nuclear scientists, or a credible mechanism for clarifying the possible military dimensions of Iran’s past efforts (to reveal how Iran kept parts of its nuclear program hidden for so long), compliance will be very hard to verify.

• Third, only Iran and IAEA inspectors are privy to the actual protocols, standards and metrics used to verify overall compliance — these details are included in undisclosed bilateral agreements between Tehran and the UN agency.

There are no American nuclear experts on the IAEA’s inspection team, which means U.S. officials will have to rely on intelligence agencies and periodic briefings by IAEA inspectors summarizing their interpretation of Iran’s compliance.

Congressional leaders on both sides are asking legitimate questions about whether this level of transparency is sufficient for them to make informed judgments about compliance and the wisdom of sanctions relief.

• Fourth, difficult arms control negotiations are usually settled with perfectly crafted language that is just vague enough to appease both sides.

Consequently, key provisions on transparency, timely access to “undeclared” sites, and prerequisites for sanctions relief will remain open to interpretation.

Even if Iranian officials have absolutely nothing to hide with respect to their nuclear ambitions, officials in Tehran are unlikely to provide the level of access congressional leaders will deem necessary to verify Iran’s (or the IAEA’s) claims.

Iran’s refusal to comply with strict interpretations of the Additional Protocol is perfectly understandable; it has legitimate reasons to be concerned about spying and leakage of sensitive non-nuclear military intelligence.

Nevertheless, lingering suspicions about clandestine programs, reinforced by Iran’s long history of non-compliance, will affect congressional support for sanctions relief.

Iran, China and Russia will interpret any delay as a clear violation of the agreement and subsequently defend Tehran’s decision to prevent inspectors from doing their job.

Unless the security dilemma at the centre of these transparency puzzles can be resolved, the same anxieties, domestic pressures and misperceptions that undermined inspections regimes in Iraq and North Korea will slowly overwhelm the Iran deal as well.

Proponents of the deal are right — now that the JCPOA has been signed, there are no credible alternatives; it is arguably the best deal negotiators could get, all things considered.

But these facts will not resolve the deal’s central problems, nor do they make it any easier to obtain congressional endorsement.

Frank P. Harvey holds the Eric Dennis chair of government and politics at Dalhousie University and is a research fellow with the Canadian Global Affairs Institute.